I, too, force my kids outside. Again, I know it isn't AP, but getting them outside can be next to impossible some days. Once they've been out for a few minutes, they generally don't want to come back in again, so I really don't feel bad about it.
For a tent, I just string a couple clotheslines across the deck, and give the kids some old sheets and blankets to hang over the lines to make their tent. We have a play tent (purchased), but if they have to make their own tent, they spend twice as long on that activity. And they really do like making their own space.
We also have a couple outdoor bags. They are just backpacks that we've filled with fun things for ourdoors - ours include bug catchers, magnifying glasses, bionoculars, walkie talkies, pirate spy glasses and eye patches, ninja costumes, super balls, string, little shovels, etc. I put one bag in each child's hand before I herd them out the door.
Sometimes I set up a treasure hunt for them. I'll hide a couple suckers or another treat somewhere in the yard. Sometimes I'll give them a series of clues to find the treat - other times I'll draw a pirate's treasure map for them.
We also only do popcicles outside, and if the weather is nice and friends come over, I shoo them all outside.
ETA: We just came in from outside, so I remembered a few more things from the bags that I'd forgotten the first time - old nature guides (insect identification guides, bird id guides, etc.) that I pick up at the used book store, rubber bands and twist ties, small frisbees and balls, small garden tools.
Also, for an activity for your older child, would you feel comfortable letting him do a building project outside when he isn't playing with the younger child? This would take some training first, but DS1 (7) learned how to use a few different hand tools (hand saw, hammer, screw driver, sanding block) this year in boy scouts. I have a pail out in the garage with some soft wood scraps and his tools, safety goggles, some nails and some screws, paint and paint brushes. He is careful when he uses his tools, so I feel fine letting him work on little projects outside.
Finally, does he like paper airplanes or paper boats? Both of my boys are crazy about folding paper airplanes and flying them outside. There are some great paper airplane kits at Borders you can get with fancy paper and instructions for all different shapes of planes. If you can teach them how to fold paper boats, they could also float these in the kiddie pool or even a bucket. My boys like to make an entire flotilla and then sink it with rocks and sticks. You can also make some pretty nice rafts by tying or twist-tieing sticks you find outside together. Again, these projects are great because they take a while for the kids to make the boats, and then there is play time after that. You'll probably have to spend time with them to teach them how to make the planes or boats at first, but once they have the concept down, they'll come up with designs of their own.
Edited by Owen'nZoe - 4/11/11 at 11:12am